This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting pigments without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-inhibiting pigments based on trivalent and tetravalent cobalt and methods of making and using the same.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion. Aluminum, zinc, iron, magnesium, titanium and their alloys tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials. The high strength 2000 and 7000 series of aluminum alloys are used extensively in aircraft and are very sensitive to corrosive attack. Materials such as steels and carbon fibers with higher redox potentials will form a galvanic couple in water and promote corrosive attack when located near light metal alloys such as aluminum.
A bare metal surface or one that has been conversion coated, phosphated, sealed, rinsed, or otherwise treated will be protected by the application of a primer paint with a corrosion inhibiting pigment. As used herein, the term “pigment” means chemically active compounds with the ability to inhibit corrosion at a distance, rather than simple colorants or opacifiers. Oxidative compounds that are effective as corrosion inhibitors tend to be highly colored and/or opaque. An effective corrosion inhibiting pigment has throwing power and can protect exposed base metal in a scratch or flaw by oxidizing and passivating it at a distance during aqueous corrosion when dispersed in a suitable carrier phase. These compounds are usually solids or liquids that are typically dispersed in a liquid carrier or binder system such as a paint or wash. These compounds may also be used to help inhibit corrosion without a significant liquid carrier using an integral binder and/or a low-volatile application method. Barrier layer formers such as sol-gel coatings or polymeric films are also used, but they tend to have no inherent oxidizing character and no appreciable throwing power and fail to protect the metal surface when the film is breached.
Pigments that contain hexavalent chromium (CrVI) compounds are the de facto standard for high-performance corrosion inhibiting paints and coatings for metal protection and are a typical corrosion inhibitor used to protect aluminum, zinc, magnesium, iron, titanium, copper and their alloys. Zinc and strontium chromate pigments are typically used, although calcium and magnesium chromates have also seen some limited use as pigments. The coating vehicles of these pigments include alkyd-type primers, acrylic primers, and elastomeric sealants, among others. Some transition metal chromate pigments (e.g., complexed with copper, iron, manganese, or cobalt) and organic chromate pigments (e.g., bound with nitrogenous compounds such as guanidinium) have been used in protective coating systems. Barium or lead chromates have been used more as colorants than as corrosion inhibitors. Variations in chromate speciation (i.e., what the chromate ions are bound to) will result in significant differences in protection when used as corrosion-inhibiting pigments.
A clear correlation between performance and solubility of chromate pigments has been shown. However, oxidizing chromates can be dangerous to use as corrosion inhibitors if they are not delivered in sufficient quantity in a timely manner to the location of a coating breach. The chromate composition was far more important to the corrosion inhibiting performance of the primer film than the organic coating composition.
A principle use of zinc and strontium chromate pigments is in wash- or etch-primer formulations for aluminum protection. Wash- or etch-primers, which have been used since the 1940s, represent one of the harshest application conditions for chromate pigments. Wash-primers are applied to metal surfaces under acidic conditions where the primer is cured as a corrosion inhibiting film. Chromate pigment powders dispersed in an alcohol/resin base mixture are combined with an aqueous phosphoric acid diluent solution. The acid roughens the metal surface and initiates cross-linking of the resin to form a pigment-filled polymeric film. The chromate pigment may also be dispersed in other carriers that are not as harsh as the wash primer. However, if a corrosion-inhibiting pigment can survive the harsh conditions of acid diluent, then it can usually be successfully incorporated within other paint, polymeric, or barrier film systems for corrosion inhibition.
An important use of chromate pigments is in coil coating formulations for steel, zinc-coated steel, or aluminum sheet stock. Coil coatings can represent a challenging application environment for pigments in that cure temperatures for these paints can exceed 100° C. Corrosion-inhibiting pigments for these applications must exhibit both throwing power to inhibit corrosion and be thermally stable at elevated temperatures when incorporated into the paint.
Significant efforts have been made in government and industry to replace CrVI with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory considerations. An effective replacement for hexavalent chromate pigment needs to have throwing power for self-healing coating breeches. Throwing power is the ability of a highly oxidized compound, such as hexavalent chromium, to oxidize and passivate the exposed bare metal in a small scratch or flaw.
A number of materials have been introduced as corrosion-inhibiting replacement pigments for hexavalent chromium-based compounds. Commercially available corrosion inhibiting pigments including compounds such as molybdates, phosphates, silicates, cyanamides, and borates that have no inherent oxidizing character have been used as alternatives to chromate pigments. Coatings that contain these materials can effectively inhibit corrosion as barrier films until the coating is breached, as by a scratch or other flaw. Films or coatings that do not contain oxidizing species can actually enhance corrosion on a surface after failure due to the effects of crevice corrosion.
Cobalt is one non-toxic, non-regulated metal which has been considered as a chromium replacement. Cobalt (like chromium) exhibits more than one oxidation state (Co+2, Co+3, and Co+4). In addition, the oxidation-reduction potential is comparable to that of CrVI in acidic solutions. For example, in acid solution:Co+3+e−=>Co+2+1.92 VCr+6+3e−=>Cr+3+1.36 V
The CoIII ion is a very good oxidizing species with an oxidation-reduction potential of +1.92 V (at pH 0). The hydroxyl and oxygen liberated from water when CoIII is reduced will oxidize nearby bare metal. This results in a passivated metal surface if sufficient oxygen is released. The potential required to reduce trivalent cobalt to divalent cobalt is only 0.26 volts greater than that needed to add three electrons to reduce CrVI to trivalent chromium (CrIII). CoII is formed during corrosion inhibition by the oxidation of base metal in the presence of CoIII and water. CoII is similar to CrIII in that neither is particularly effective as redox-based corrosion inhibitors.
A number of pigments using cobalt have been reported in the literature over the years, but none approach the general performance or utility of CrVI-based pigments. Trivalent cobalt oxide Co2O3 or Co3O4) and hydroxide (Co(OH)3) pigments have been disclosed for corrosion protective coatings. However, the coatings formed provide only limited protection and do not approach the benefit derived from the use of hexavalent chromium.
A number of compounds have been described as corrosion-inhibiting agents, including organic mercapto and thio compounds, cyclic tetraaza compounds, aminophosphonic acid, and triazinedithiols and triazinetrithiols. Other compounds havebeen described as corrosion inhibiting when complexed with cobalt, typically in the divalent charge state. Among these compounds are porphyrin derivatives, tetraaza organic compounds, phosphoric and phosphonic acids, naphthenates, amidosulfonic acids, and amino acids. However, the pigments formed from these compounds provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium.
In addition, the formation of cobalt-containing pigments in which the cobalt is complexed with ligands such as hydrazones, —O bidentates, azomethines, phthalocyanines, azo and disazo complexes, N—S bidentates, oximes, tetraaza complexes, porphyrins, 1,2-dithiolates, and semicarbazones, has also been described. However, these compounds do not use trivalent cobalt and are not used for anticorrosive applications.
U.S. Pat. Nos. 4,096,090 and 4,012,195 to Noack describe catalyzed hydrazine compositions that contain cobalt and act as corrosion inhibitors. For example, U.S. Pat. No. 4,096,090 describes compositions that contain: a) a hydrazine compound; b) a cobalt organometallic compound derived from the reaction of cobalt (II) hydroxide with unsubstituted and substituted orthodihydroxy aromatic compounds and unsubstituted and substituted ortho-aminohydroxy aromatic compounds. Noack observed that 25% of the dissolved oxygen was “removed” in 0.1 minutes, and 95% in 0.3 minutes. Likewise, U.S. Pat. No. 4,012,195 describes a composition containing: a) a hydrazine compound; b) an organometallic complex formed from the reaction of a salt of cobalt and one or more ligands selected from the group consisting of unsubstituted and substituted orthodiamino aromatic compounds, unsubstituted and substituted orthodihydroxy aromatic compounds and unsubstituted and substituted orthoaminohydroxy aromatic compounds. The cobalt is present in the divalent oxidation state. These compositions are also claimed to be oxygen scavengers. Although not specified in the Noack patents, this “oxygen scavenging” activity is the result of dissolved oxygen oxidizing the divalent cobalt to a higher oxidation state. While the combination of these organic compounds with divalent cobalt inhibited corrosion once they “scavenged oxygen”, Noack failed to realize that the important constituent of these inhibitor compositions was a trivalent cobalt ion, stabilized by the organic additives.
European Patent Application EP 634,460 to Bamber, et al. describes the use of organic phosphoric or phosphonic acids optionally in conjunction with cobalt for anticorrosive pigments. Further, Bamber, et al. teach a desired solubility of 2 grams per liter or lower as needed at 20° C. The oxidation state of the cobalt is not specified, nor is there described any process that would increase the oxidation state of the cobalt to the trivalent or tetravalent oxidation state. Organic phosphonates or phosphorates are less desirable valence stabilizers for trivalent or tetravalent cobalt, because stabilization of these desired oxidation states are typically not possible using these agents by themselves. This means a resultant oxidation state of +2 for the cobalt and, therefore, pigments derived from these formulations will exhibit low anticorrosive properties.
UK Patent Applications GB 2,138,796 and 2,139,206, as well as German Patent DE 3,309,194 to Fuchs, et al. also describe the use of cobalt complexes in combination with hydrazine to “scavenge oxygen” in boiler feedwaters and therefore inhibit corrosion. These compositions utilize trivalent cobalt precursors such as Na3Co(NO2)6 and Co(NH3)5Cl3 as the cobalt source. German Patent DE 3,309,194 and UK Patent Application GB 2,139,206 utilize 2-acetamino-4-nitrophenol and/or 2-amino-4-nitrophenol as the third constituent of the compositions, whereas UK Patent Application GB 2,138,796 utilizes pyrogallol as the third constituent. These patents do not recognize the important corrosion-inhibiting properties of trivalent cobalt complexes. These compositions are also “oxygen scavengers”, because reacting Co+3 (in the precursors) with hydrazine (a strong reducing agent) in the presence of the third constutuent results in Co+2 complexes.
Similarly, U.S. Pat. No. 4,479,917 to Rothgery and Manke describe the use of aminoguanidine compounds, optionally in conjunction with cobaltous (divalent cobalt) hydroxide for anticorrosion purposes. These compounds are also said to act as “oxygen scavengers”. However, the use of trivalent cobalt in conjunction with this compound is not disclosed.
European Patent Application EP 675,173 to Glausch, et al. describes anticorrosive pigments that are derived from cobalt-containing phthalocyanine and tetraazatetradecane derivatives. However, these pigments are not reported to contain trivalent or tetravalent cobalt.
To date, no truly effective replacements have been developed for pigments based on CrVI. Accordingly, the need remains for improved corrosion-protective pigments composed of currently unregulated and/or nontoxic materials which have an effectiveness, ease of application, and performance comparable to current CrVI pigment formulations, and for methods of making and using the same.